disable the readonly state while rna property callbacks run.

source/blender/python/intern/bpy_props.c

@@ -185,9 +185,14 @@ void bpy_prop_update_cb(struct bContext *C, struct PointerRNA *ptr, struct Prope
 	PyObject *args;
 	PyObject *self;
 	PyObject *ret;
+	const int is_write_ok= pyrna_write_check();
 
 	BLI_assert(py_data != NULL);
 
+	if(!is_write_ok) {
+		pyrna_write_set(TRUE);
+	}
+
 	bpy_context_set(C, &gilstate);
 
 	py_func= py_data[BPY_DATA_CB_SLOT_UPDATE];
@@ -216,6 +221,10 @@ void bpy_prop_update_cb(struct bContext *C, struct PointerRNA *ptr, struct Prope
 	}
 
 	bpy_context_clear(C, &gilstate);
+
+	if(!is_write_ok) {
+		pyrna_write_set(FALSE);
+	}
 }
 
 static int bpy_prop_callback_check(PyObject *py_func, int argcount)

source/blender/python/intern/bpy_rna.c

@@ -323,11 +323,20 @@ int pyrna_write_check(void)
 {
 	return !rna_disallow_writes;
 }
+
+void pyrna_write_set(int val)
+{
+	rna_disallow_writes= !val;
+}
 #else // USE_PEDANTIC_WRITE
 int pyrna_write_check(void)
 {
 	return TRUE;
 }
+void pyrna_write_set(int UNUSED(val))
+{
+	/* nothing */
+}
 #endif // USE_PEDANTIC_WRITE
 
 static Py_ssize_t pyrna_prop_collection_length(BPy_PropertyRNA *self);

source/blender/python/intern/bpy_rna.h

@@ -183,6 +183,7 @@ PyObject *pyrna_math_object_from_array(PointerRNA *ptr, PropertyRNA *prop);
 int pyrna_array_contains_py(PointerRNA *ptr, PropertyRNA *prop, PyObject *value);
 
 int pyrna_write_check(void);
+void pyrna_write_set(int val);
 
 int pyrna_struct_validity_check(BPy_StructRNA *pysrna);
 int pyrna_prop_validity_check(BPy_PropertyRNA *self);